Alkaline storage batteries such as nickel-cadmium storage batteries and nickel-metal hydride storage batteries have high capacity and are expected to be utilized for various applications. Particularly in recent years, also in such an application as main power source for portable electronic devices and other electronic devices, or backup power source, alkaline storage batteries have been assumed to be used. Studies have been made for utilization of alkaline storage batteries in those applications as auxiliary power source for batteries that have been charged and utilization as emergency power source in disaster areas.
In alkaline storage batteries, a nickel oxide, including nickel oxyhydroxide and nickel hydroxide, is typically used as a positive electrode active material. As shown in the formula below, during charge, the nickel hydroxide is converted to nickel oxyhydroxide; during discharge, the nickel oxyhydroxide is converted to nickel hydroxide.Negative electrode: MH+OH−M+H2O+e−Positive electrode: NiOOH+H2O+e−Ni(OH)2+OH−Whole reaction: NiOOH+MHNi(OH)2+M  [Chem. 1]
(In the formulas, M represents a hydrogen storage alloy)
In view of improving the characteristics of alkaline storage batteries, various approaches have been made. For example, Patent Literature 1 suggests that the internal pore volume of the nickel hydroxide forming an active material layer be 0.1 mL/g or less, and the particle diameter thereof be from 20 to 40 μm, in view of improving high-rate discharge characteristics.
Patent Literature 2 suggests increasing the tap density and the bulk density of the nickel hydroxide particles, in order to improve the packing density of the positive electrode. Patent Literature 3 suggests that the space volume having a pore radius of 3 nm or more of the nickel hydroxide active material be 20 to 70% of the total space volume, in view of improving the active material utilization rate in the early stage of charge and discharge.